Page 5 - GSA Today December 2022

can be simplified to Equations 1–4, show- depth in reconstructed soils as they would alteration add elements and mass, so have
ing that each mole of oxide consumed 2 have been before burial compaction and positive strain and mass transfer. Moles of
moles of CO : metamorphism (Sheldon, 2006). CO used to displace alkali and alkaline
2 2
Original soils can be reconstructed from earths during weathering assessed by tau
MgO + 2CO + HO Mg 2 2 HCO , (1) paleosols by estimating compaction due to analysis (Equations 7–8) can be used to cal-
−
2
3
2
burial by overburden (C as %) from total culate soil CO (ppm) consumed by the
2
2+
CaO + 2CO + HO Ca + 2HCO , (2) depth of burial (B in km) and suitable phys- whole profile during its formation using
−
2
3
2
ical constants, in this case taken from Equations 9–11 (modified from Sheldon,
Na O + 2CO + HO 2Na + 2HCO , (3) Aridisols (Sheldon and Retallack, 2001): 2006). Components of these calculations are
+
−
3
2
2
2
areas under the curves of depletion of bases
.
051 100
KO + 2CO + HO 2K + 2HCO . (4) C . (6) or phosphorus in reconstructed paleosol
+
−
2
2
3
2
.
049 profiles, calculated for the whole profile for a
Losses of these elements from soils on a B 1 square centimeter of surface area of the pro-
molar basis is a proxy for moles of CO con- e 027. file (Fig. 2):
2
sumed by soil over its time of formation F
(Sheldon, 2006). Whole profile loss can be pCO 2 , (9)
D
P
envisaged as the area under the curves in Tau analysis of paleosols (Brimhall et al., A K CO 2 2
CO
κ
mole fraction alkali and alkaline earth deple- 1992) calculates mole fraction mass trans- 1000 L
tion for decompacted paleosols (Fig. 2). port (τ ) of a mobile element and mole frac-
j,w
Dissolution of apatite as a source of P can tion strain (ε ) of the profile during soil C
i,w
be reduced to Equation 5, in which 1 mol of formation using an immobile element from F 2 p jp , ZD jw, jw z Z , (10)
,
CO in aqueous solution liberates 3 moles of the parent material (Ti used here). Equations 100 Z 0
2
soluble phosphate from apatite: 7–8 for mass transport and strain include
bulk density (ρ in g.cm ) and oxide assay G 5 C jp , ZD jw, Z . (11)
–3
,
Ca PO 4 3 OH + CO (5) (C in wt%) for successive samples (sub- p 100 Z 0 jw z
5
2
5Ca + 3PO 3− + HCO − . scripts i, j) of weathered material (subscript
2+
3
4
w) and parent material (subscript p) of a Variables and constants for these calcula-
This is a simplification of four intermedi- single paleosol profile: tions besides those needed for Equations
ate apatite dissolution reactions and other 6–8 are F (mol CO .cm ) = summed molar
–2
2
intermediate reactions producing carbonic C mass transfer loss of CaO, MgO, Na O, and
2
acid from CO in solution (Dorozhkin, 2012). iw p j p 1, (7) K O using Equation 9; G (mol CO .cm ) =
,
–2
,
2
2
2
Actual phosphate procurement in soils from w C j w summed molar mass transfer loss of P using
,
relatively insoluble apatite is catalyzed by a Equation 10; Z (cm) = depth in soil repre-
variety of carbon-based acid moieties, such C sented by analysis corrected for compaction
.
as acetic and oxalic acid with higher mole jw w j w iw 1 1. (8) using Equation 10; A (years) = duration of
,
,
fractions of carbon (Neaman et al., 2005). p C j p soil formation using Equations 12 and 13;
,
Another complication is that Archean apatite K (mol./kg.bar) = Henry’s Law constant for
CO
2
dissolution also may have been partly Soils and paleosols lose mass with weath- CO (=0.034, range 0.031–0.0045); P (cm) =
2
achieved by strong sulfuric acid, rather than ering and so have negative strain (ε < 0), mean annual precipitation using Equation
i,w
weak carbonic acid (Retallack, 2022c). and also lose nutrient cations and silica, so 13; κ (s.cm .[mol.year] ) = seconds per year
3
–1
Again, this is based on mass transfer, includ- have negative mass transfer (τ < 0). In con- divided by volume per mole of gas at stan-
j,w
ing volume loss during soil formation with trast, sediment accumulation and diagenetic dard temperature and pressure (=1430);
Figure 2. Base and phosphorus depletion in a 550 Ma paleosol from South Australia as an example of output data for each reconstructed paleosol.
Parent material was chosen on the basis of petrographic, titania, and sesquioxide similarity detailed elsewhere (Retallack, 2013).
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